Research On Distributed Testing Method Of Fiber Bragg Grating Based On Short Pulse

Fiber Bragg grating(FBG)sensors have many advantages,such as low cost,small size,strong multiplexing ability,and are one of the most studied types of fiber sensors.In most grating sensing applications,the measured physical quantity is the overall average of its change over the entire grating area,which cannot be applied to some measurements with high spatial resolution requirements such as crack detection.Therefore,the realization of fiber grating distributed sensing is of great significance.In this dissertation,the distributed test method of time-frequency analysis is used to study the transmission characteristics of picosecond and nanosecond pulses in FBG,and the corresponding distributed test experiments are carried out.Firstly,the coupling mode theory and the transmission matrix method of FBG are introduced,and the calculation steps of FBG reflection spectrum and pulse response are given.The FBG characteristics and pulse transmission characteristics at different modulation amplitudes and different FBG lengths are numerically simulated and discussed.The effects of local strain on the spectrum and impulse response of gratings with different reflectivity are shown.Simulation results show that weak gratings are more suitable for distributed measurement.Secondly,the distributed measurement method of FBG based on picosecond pulse is studied.The effects of different strain size,strain position and strain length on the characteristics and pulse response of FBG are simulated.Because the bandwidth of FBG is much smaller than that of picosecond pulse light source,then the pulse transmission characteristics are studied when the center wavelength of FBG is different from the relative position of the center of light source.The simulation results show that the reflected pulse energy varies with the relative position of the light source spectrum and the FBG spectrum,and the rules of the system resolution,sensitivity and other parameters changing with the relative position of the FBG spectrum and the light source spectrum are summarized.As a comparison,a distributed measurement method based on microwave photon filter is proposed and the timefrequency response of the system is simulated.Finally,the distributed measurement method of FBG based on nanosecond pulse is studied.The spectra of cascaded FBGs and single FBG are numerically simulated,and the FBG spectra and pulse response characteristics of different strain sizes and positions are analyzed.As the bandwidth of FBG is larger than that of nanosecond pulse light source,the sensitivity and dynamic range of the system change with the relative position of FBG spectrum and light source spectrum.A distributed FBG measurement scheme based on nanosecond pulses is proposed,and the impulse responses in two cases of strong FBG cascade and weak FBG cascade are simulated,and experimental verification is performed.